Nanomaterials refer to the materials whose size unit does not reach the scale of micrometer. They are characterized by having a much larger surface area and relatively higher surface energy than the earlier materials. Such a large surface area and high surface energy greatly influence the physical properties of the material, and make it to have very different characteristics from the corresponding previously known materials.
For example, silver (Ag) nanoparticles may have very different melting points depending on their diameters. That is, they represent a melting point of about 200˜300° C. at a diameter of about 20 nm, which is greatly different from about 960.5° C., the previously known melting point of silver (Ag). Furthermore, it is well known that CdTe—a kind of semiconductor materials—shows a very different color of fluorescence in the state of a nanoparticle even by the difference of 1 nm of the particle size.
As such, even though the nanomaterials consist of the same component, it may be expected that they have different crystal size, surface area, color, distribution of crystal face, etc. depending on their particle size, shape, etc. This is why the nanomaterials having different particle size, shape, etc. may be expected to exhibit quite different characteristics despite they consist of the same component.
Thus, it is very important to control the size, shape, composition, etc. of nanomaterials to achieve desired characteristics of the nanomaterials.
On the other hand, since metals have a variety of catalytic activities and high strength as well as an excellent thermal conductivity and electrical conductivity, they have been used as very important materials in the industrial field. In particular, when such metals are obtained as nanomaterials, they can overcome the limits of the earlier metals and show new physical properties. Thus, recently, various researches for the metal nanomaterials have been made.
Among such metal nanomaterials, since nanomaterials or nanostructures containing a noble metal such as platinum (Pt), palladium (Pd), ruthenium (Ru), cobalt (Co), iridium (Ir), rhodium (Rh), etc. exhibit an excellent activity as a catalyst or a sensor, they are widely used as a catalyst in various reaction processes, fuel cells, sensors, etc.
The activity of such a nanomaterial containing a noble metal may be widely different according to the composition of the catalytically active material (e.g., noble metal component) constituting the nanomaterial, surface area of the catalytically active material that can interact with the reactants, crystal structure or size of the catalytically active material, crystal face of the catalytically active material that is exposed on the surface of the catalyst, etc.
In the case of the previously known nanomaterials or nanostructures containing a noble metal, however, they usually have a shape of particle or one-dimensional wire. Unless the content of the catalytically active material (e.g., noble metal component) increases to some level or higher, there is a limit to increasing the surface area of the catalytically active material that can interact with the reactants. Thus, there is a limit to securing an excellent catalytic activity of the nanomaterial containing a noble metal.
It is also true that the earlier nanomaterials or nanostructures containing a noble metal have a limit in selectively exposing the specific crystal face of the catalytically active material (e.g., noble metal component) on the surface of the catalyst due to their limit in the shape, etc.
Therefore, unless the content of the catalytically active material, that is typically expensive, increases to some level or higher, it is difficult to make the catalytic activity of the noble metal-containing nanomaterials excellent, and therefore a catalyst having an excellent activity can hardly be economically and effectively obtained.